A variety of air purification systems have been developed to protect people from hazardous air contaminants. Among these air purification systems are a wide range of air purifying respirators that are designed to filter out or sorb contaminants present in the air. Typically these air purifying respirators contain a filter media, a filter body, or some combination of filter media and filter body. Upon use of the respirator, the contaminants become absorbed by the filter media or attached or trapped by the filter body. Eventually, the filter media or filter body becomes saturated and the ability of the respirator to remove the hazardous air contaminants begins to diminish.
During extended exposure to an environment containing hazardous air contaminants, such as, for example, continuous or repeated worker exposure to such environments, techniques are necessary to determine the useful service life of a respirator. One technique that has been developed is based upon the time in service for a respirator. In this technique, respirators or the air purifying filters are replaced after a certain period of time in service. However, this technique does not take into account variations in contaminant level or flow rates through the respirator and therefore may result in the respirator or filter elements being changed too early (which is wasteful) or too late (which may present a danger to the user).